Many electro-optic devices exploit the free carrier dispersion effect to alter both the real and imaginary parts of the refractive index. This exploitation is used since the unstrained pure crystalline silicon does not exhibit a linear electro-optic (Pockels) effect, and the refractive index changes caused by the Franz-Keldysh effect and Kerr effect are very weak. Phase modulation in a specific region of optical devices, such as Mach-Zehnder modulators, total-internal-reflection (TIR)-based structures, cross switches, Y-switches, ring resonators, and Fabry-Perot resonators may be used to modulate the output intensity.
Free carrier concentration in electro-optic devices can be varied by injection, accumulation, depletion, or inversion of carriers. Most of the devices investigated to date exhibit some common features, such as requiring long interaction lengths (for example, 5-10 mm) and injection current densities higher than 1 kA/cm3 in order to obtain a significant modulation depth.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.